1. Field of the Invention
The present invention relates to a cutting tool made of a surface-coated cemented carbide alloy (hereinafter referred to as a coated cemented carbide tool) whose cutting edge portion exerts an excellent heat-resistant plastic deformability when various types of steel and cast iron are cut under high-speed cutting conditions accompanied with high heat generation.
2. Description of the Related Art
There has conventionally been known a coated cemented carbide tool comprising, on the surface of a substrate made of a tungsten carbide (hereinafter referred to as WC)-based cemented carbide alloy (hereinafter referred to as a cemented carbide substrate), a hard coating layer composed of the following layers (a) to (c) deposited thereon:
(a) a Ti compound layer, as a lower layer, formed by chemical vapor deposition (hereinafter referred to as xe2x80x9cCVDxe2x80x9d) and/or physical vapor deposition (hereinafter referred to as xe2x80x9cPVDxe2x80x9d) which has an average thickness of 3 to 20 xcexcm and is made of one layer or a plurality of two or more layers among a layer of carbide of Ti (hereinafter referred to as TiC), a layer of nitride of Ti (hereinafter referred to as TiN), a layer of carbonitride of Ti (hereinafter referred to as TiCN), a layer of carboxide of Ti (hereinafter referred to as TiCO) and a layer of carbonitroxide of Ti (hereinafter referred to as TiCNO);
(b) an aluminum oxide (hereinafter referred to as Al2O3) layer, as an upper layer formed by CVD which has an average thickness of 3 to 15 xcexcm and has an xcex1-type crystal structure; and
(c) if necessary, a TiN layer having an average thickness of 0.5 to 2 xcexcm, as a surface layer, is deposited on the upper layer for the purpose of identification of the cutting edges before and after cutting operations because of its golden color tone. Also, it has been known that this coated cemented carbide tool may be used for both continuous and interrupted cutting operations of various types of steel and cast iron.
Also, it has been known that the Ti compound layer and the Al2O3 layer, which constitute the hard coating layer of the coated cemented carbide tool described above, generally have a granular crystal structure and, as described in Japanese Patent Application, First Publication No. Hei 6-8010, and Japanese Patent Application, First Publication No. Hei 7-328808, the TiCN layer constituting the Ti compound layer is formed by CVD at a moderate temperature within a range from 700 to 950xc2x0 C. using a mixed gas containing an organic carbonitride as a reaction gas in a conventional CVD apparatus, thereby providing the layer with a crystal structure grown longitudinally for the purpose of improving the toughness of the layer itself.
With recent increases in demands for labor saving and energy saving as well as cost reduction in cutting operations, cutting operations tend to be conducted at high speed, along with the development of high performance cutting machines. When a conventional coated cemented carbide tool is used in continuous cutting or interrupted cutting of various types of steel and cast iron under normal conditions, no problem arise. However, when the coated cemented carbide tool is used in a high-speed cutting operation, thermoplastic deformation, which can cause abnormal wear at the cutting edge portion, is liable to occur due to high heat generated during the cutting operation. As a result, the process of wear is accelerated and failure occurs within a relatively short time.
From aforementioned point of view, the present inventors have studied about the conventional coated cemented carbide tool in order to develop a coated cemented carbide tool, which exerts an excellent thermoplastic deformation resistance during high-speed cutting operations, thus yielding the following results (1) to (3).
(1) When the Ti compound layer, as a lower layer, is formed on the surface of a cemented carbide substrate under normal deposition conditions and an Al2O3 layer having a xcexa- or xcex8-type crystal structure is formed also under normal deposition conditions and then the resulting material is subjected to a heat treatment in this state, preferably, in an Ar atmosphere under the conditions of a temperature of 1000xc2x0 C. or higher for a predetermined time, the xcexa- or xcex8-type crystal structure is converted into an xcex1-crystal structure. As a result, cracks formed during the heat transformation are uniformly dispersed and distributed in the resulting heat transformed xcex1-type Al2O3 layer and the heat transformed xcex1-type Al2O3 layer operates as a heat-insulating layer for high heat generated during high-speed cutting operations due to an action of a large number of cracks which are present in the heat transformed xcex1-type Al2O3 layer, and also suppresses high heat from being transferred to the cemented carbide substrate. Consequently, thermoplastic deformation of the cutting edge portion is markedly suppressed and the occurrence of abnormal wear is prevented, and therefore the cutting edge portion exhibits a normal wear pattern, thus enabling cutting operations for a long period.
(2) In a coated cemented carbide tool comprising a hard coating layer composed of the heat transformed xcex1-type Al2O3 layer, as an intermediate layer, and an xcex1-type Al2O3 layer, as an upper layer, deposited on the surface of the intermediate layer also under normal deposition conditions, some portion of the deposited Al2O3 sufficiently enters into cracks formed during the heat transformation at the interface with the heat transformed xcex1-type Al2O3 layer, thereby making it possible to maintain the cracks formed during the heat transformation in a markedly stable state, thus enabling a cutting operation for a long period without causing chipping even if high-speed cutting is conducted under the interrupted conditions.
(3) In the heat transformed xcex1-type Al2O3 layer formed by heat transformation of the xcexa-type Al2O3 layer, since the basal plane of the hexagonal crystal of the respective crystals constituting the layer exhibits a unique orientation, which is generally in parallel with the film growth surface, such that the film itself has excellent wear resistance, the layer exhibits excellent cutting performances, along with the heat-insulating effect of the film.
The present invention has been completed based on the results described above and provides a coated cemented carbide tool, which exerts an excellent thermoplastic deformation resistance during the high-speed cutting operation, the cutting tool comprising, on the surface of a cemented carbide substrate, a hard coating layer including the following layers (a) to (c) or (a) to (d):
(a) a Ti compound layer, as a lower layer, which has an average thickness of 0.5 to 20 xcexcm, preferably 3 to 15 xcexcm, and more preferably 5 to 10 xcexcm, and is made of one layer or a plurality of two or more layers formed by vapor deposition process, among a TiC layer, a TiN layer, a TiCN layer, a TiCO layer and a TiCNO layer;
(b) a heat transformed xcex1-type Al2O3 layer, as an intermediate layer, which has an average thickness of 1 to 25 xcexcm, preferably 3 to 15 xcexcm, and more preferably 5 to 10 xcexcm, formed by heat transformation of a vapor deposited xcexa- or xcex8-type Al2O3 layer;
(c) an Al2O3 layer, as an upper layer, formed by vapor deposition process which has an average thickness of 0.3 to 10 xcexcm, preferably 0.5 to 5 xcexcm, and more preferably 0.5 to 2 xcexcm, and an xcex1-type crystal structure; and
(d) if necessary, at least one layer of TiN, TiC or TiCN, as a surface layer (d), formed by vapor deposition process which has an average thickness of 0.1 to 5 xcexcm, preferably 0.3 to 4 xcexcm, and more preferably 0.5 to 2 xcexcm.
The Al2O3 layer having the heat transformed xcex1-type crystal structure preferably has a structure in which cracks formed during the heat transformation are uniformly dispersed and distributed.
The average thickness of each constituent layer of the hard coating layer of the coated cemented carbide tool of the present invention is limited for the following reasons.
(a) Lower Layer (Ti Compound Layer)
The Ti compound layer itself has the toughness (strength) and the hard coating layer is provided with the toughness by the presence of the Ti compound layer, and also strongly adheres to any of the cemented carbide substrate and the heat transformed xcex1-type Al2O3 as the intermediate layer, thereby to exert an effect of contributing to an improvement in adhesion to the cemented carbide substrate of the hard coating layer. However, when the average thickness is less than 0.5 xcexcm, the effect described above cannot be sufficiently exerted. On the other hand, when the thickness exceeds 20 xcexcm, chipping is liable to occur at the cutting edge portion. Therefore, the average thickness of this layer is preferably within a range from 0.5 to 20 xcexcm.
(b) Intermediate Layer (Heat Transformed xcex1-type Al2O3 Layer)
As described above, the heat transformed xcex1-type Al2O3 layer has an effect of preventing high heat generated during high-speed cutting operation due to a function of a large number of cracks, which are uniformly dispersed and distributed in the layer, from being transferring to the cemented carbide substrate, thereby suppressing thermoplastic deformation. However, when the average thickness is less than 1 xcexcm, the effect described above cannot be sufficiently exerted. On the other hand, when the average thickness exceeds 25 xcexcm, cracks formed during the heat transformation, which are present in the intermediate layer, can cause chipping. Therefore, the average thickness of this layer is preferably within a range from 1 to 25 xcexcm.
(c) Upper Layer (xcex1-type Al2O3 Layer)
The upper layer has a function of sufficiently entering into cracks formed during the heat transformation at the interface with the heat transformed xcex1-type Al2O3 layer, thereby making it possible to maintain the cracks formed during the heat transformation in a markedly stable state. However, when the average thickness is less than 0.3 xcexcm, the function described above cannot be sufficiently exerted. On the other hand, when the average thickness is up to 10 xcexcm, the function can be sufficiently exerted. Therefore, the average thickness of this layer is preferably within a range from 0.3 to 10 xcexcm.
(d) Surface Layer (At Least One Layer of TiN, TiC or TiCN)
In the case in which the surface layer is formed of TiN, it is optionally formed by vapor deposition process for the purpose of identification of the cutting edges before and after cutting operation because of a golden color tone. However, when the average thickness is less than 0.1 xcexcm, a sufficient identification effect cannot be obtained. On the other hand, when the average thickness is up to 5 xcexcm, the identification effect due to the TiN layer is sufficient. Therefore, the average thickness of this layer is preferably within a range from 0.1 to 5 xcexcm taking into account economic efficiency. Even if a portion or all of the portion excluding the outermost portion of the TiN layer is replaced by the TiC layer and/or the TiCN layer, the above effect is maintained.
It is preferred to provide a Ti oxide layer, which has an average thickness of 0.2 to 5 xcexcm and meets the formula: TiOx (provided that an atomic ratio x of O to Ti is within a range from 1.2 to 1.9) as measured by an Auger electron spectroscopy at the center portion in the thickness direction, between the upper layer and the surface layer.
The Ti oxide layer has very low affinity for steel and cast iron and also has a characteristic wherein chips produced during cutting are less likely to be smeared, that is, there is surface lubricity. Consequently, abnormal damage caused by smeared chips is markedly suppressed. This effect is particularly exerted when workpieces having a high viscosity made of stainless steel and mild steel are cut.
A ratio of a peak intensity of (006) plane, I(006), to a peak intensity of (113) plane, I(113), is preferably 0.1 or more in an X-ray diffraction profile of the xcex1-type aluminum oxide layer which constitutes the hard coating layer.
A ratio of a peak intensity of (006) plane, I(006), to a peak intensity of (012) plane, I(012), is preferably 0.1 or more in an X-ray diffraction profile of the xcex1-type aluminum oxide layer which constitutes the hard coating layer.
The heat transformed xcex1-type Al2O3 layer is preferably formed in the following manner. First, in order to stably form a xcexa- or xcex8-type crystal structure without including an xcex1-type component, an Al2O3 layer is formed under the conditions of a temperature of 1000xc2x0 C. or less, preferably 970xc2x0 C. or less, and more preferably 950xc2x0 C. or less. Subsequently, in order to convert the resulting Al2O3 layer into an xcex1-type crystal structure as completely as possible, the xcexa- or xcex8-type crystal structure is subjected to a heat treatment by heating to 1020xc2x0 C. or higher, preferably 1040xc2x0 C. or higher, and more preferably 1060xc2x0 C. or higher, and being allowed to stand for a predetermined time, thereby converting into an xcex1-crystal structure.